Measuring system for determining the reactance ratio of a pair of reactive devices

ABSTRACT

By simultaneously charging a pair of capacitors from the same low voltage level to the same high reference voltage level, the two charge time durations will be functions of and will be proportional to the capacitances of the two capacitors. A comparison of those charge time durations will thus provide an indication of the ratio of the capacitances of the two capacitors. This is achieved by including, in the measuring system, an oscillating circuit whose frequency is determined by the capacitance of one of the capacitors. A periodically-recurring rectangular wave is then developed having pulse components with pulse widths determined by the capacitance of the other capacitor, the frequency of the rectangular wave being equal to the oscillating frequency. The duty cycle of the rectangular wave is therefore proportional to and respresents the ratio of the capacitances of te capacitors. By averaging the rectangular wave in a low-pass filter, an output signal is produced having an amplitude which is proportional to and represents the capacitance ratio.

BACKGROUND OF THE INVENTION

This invention relates in general to a measuring system for determining the ratio of a pair of reactances and in particular to a precision measuring system for comparing the electrical sizes of two capacitors to determine their ratio with a high degree of accuracy.

A variety of arrangments have been developed for measuring the capacitance of capacitors. Most of these arrangements, however, require constant operating characteristics, such as a constant supply voltage, operating temperature, etc., to obtain accurate results. In contrast, the measuring system of the present invention achieves very precise measurements at all times and is immune to any variations in operating characteristics. Moreover, this is accomplished by means of a relatively simple and inexpensive circuit arrangement. While the measuring system of the invention determines the ratio of two capacitors, it can easily be used to determine an unknown capacitance when the other capacitance is known.

SUMMARY OF THE INVENTION

The invention provides, in accordance with one of its aspects, a measuring system for comparing first and second capacitors to determine the ratio of their capacitances. The system comprises means for simultaneously charging the first and second capacitors, starting at time t_(o), from the same relatively low voltage level to the same relatively high reference voltage level. The first capacitor has a charge time constant shorter than that of the second capacitor and charges to the reference voltage level, at time t₁, before the second capacitor reaches the reference voltage level at the later time t₂, the time duration from t₀ to t₁ thereby being a function of the capacitance of the first capacitor while the time duration from t₀ to t₂ is a function of the capacitance of the second capacitor. The measuring system also comprises means for effectively comparing the two time durations and for producing, from the comparison, an output signal representing the ratio of the capacitances of the first and second capacitors.

DESCRIPTION OF THE DRAWINGS

The features of the invention which are believed to be novel are set forth with particularity in the appended claims. The invention may best be understood, however, by reference to the following description in conjunction with the accompanying drawings in which:

FIG. 1 schematically illustrates a measuring system, constructed in accordance with one embodiment of the invention, for determining the ratio of the capacitances of a pair of capacitors C₁ and C₂ ; and,

FIG. 2 depicts various signal waveforms that will be helpful in understanding the operation of the measuring system.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

Capacitors C₁ and C₂ may take a variety of different forms. The capacitance of one or both may be unknown. In either event, the measuring system will provide an output signal bearing information which indicates the ratio of the two capacitances. As one example of capacitors C₁ and C₂, they may both be included in a capacitive ceramic pressure sensor manufactured by Borg-Warner Corporation under the designation 358-522. As the pressure changes, the capacitance of one of the capacitors increases while the capacitance of the other capacitor decreases. In other words, the ratio of the capacitances varies with pressure. Hence, a measurement of the ratio will indicate the sensed pressure.

As will be apparent, the measuring system of FIG. 1 includes an oscillating circuit so that the system will operate in cyclic fashion. At time t₀ (see FIG. 2) transistor T₁ will be turned on, by the first-occurring triggering pulse in voltage waveform V_(T), and capacitor C₁ will rapidly discharge from a positive voltage, nearing the voltage V_(s) of the d-c power supply 10, to a very small positive voltage as determined by the very little voltage drop across transistor T₁. Hence, capacitor C₁ discharges to essentially zero volts as indicated by the waveform V_(c).sbsb.1 which shows the voltage across capacitor C₁. At the time t₀ when capacitor C₁ discharges to zero, the voltage at the negative input of comparator 11 will become less than (namely negative relative to) that at the comparator's positive input, to which is applied a d-c reference voltage V_(ref). produced by the voltage divider, comprising resistors 14 and 15 and diode 16, which in turn is coupled across d-c power supply 10. The voltage drop of diode 16 is equal to that introduced by transistors T₁ and T₂ when they conduct, the transistors being substantially identical in construction. In this way, the voltage drops across the transistors are effectively compensated by the voltage drop across diode 16. Providing identical transistors is easiest achieved by forming the transistors, along with most of the other elements shown in FIG. 1, on the same monolithic chip. Preferably, resistors 14 and 15 are of equal value so that the reference voltage V_(ref)., at the circuit junction of those resistors, will be essentially one-half of the voltage V_(s). In response to the lower voltage at the negative input of comparator 11 at time t₀, the output voltage of the comparator abruptly switches from its low level, or zero volts, to its high level, as indicated by voltage waveform V_(o) in FIG. 2.

At the same time t₀ that capacitor C₁ discharges to a relatively low voltage, namely to zero volts, the first-occurring triggering pulse in voltage waveform V_(T) also renders transistor T₂ conductive to cause capacitor C₂ to rapidly discharge from the d-c reference voltage V_(ref). to the same low voltage or zero level, as shown by voltage waveform V_(c).sbsb.2. That negative-going voltage on capacitor C₂ at time t₀ establishes the positive input of comparator 12 at a lower potential level than the comparator's negative input, to which is applied the d-c reference voltage V_(ref).. As a result, the output voltage of comparator 12 abruptly switches to its low, or zero volts, level as shown by waveform V_(T). Actually, switching of comparator 12 to its low output voltage level in response to discharging of capacitor C₂ is what causes the triggering pulse to terminate. When that termination occurs, both transistors T₁ and T₂ will be turned off. There is sufficient delay in the circuit to allow capacitors C₁ and C₂ to fully discharge to the low voltage or zero level before the transistors are cut off.

After the occurrence of the first triggering pulse of waveform V_(T), capacitors C₁ and C₂ will both begin to simultaneously, and slowly, charge from zero volts toward the output voltage V_(s) of d-c power supply 10, as shown by waveforms V_(c).sbsb.1 and V_(c).sbsb.2. Capacitor C₁ will charge through series-connected resistor R₁, while capacitor C₂ charges through series-connected resistor R₂. The system is designed so that regardless of the particular capacitances of the capacitors at any one time, and those values may vary over a relatively wide range in some applications of the invention, the charge time constant of capacitor C₁ will always be shorter than that of capacitor C₂. In this way, capacitor C₁ will charge to the reference voltage V_(ref)., at time t₁, before the capacitor C₂ reaches the reference voltage level at the later time t₂ as illustrated in FIG. 2. When the voltage V_(c).sbsb.1 across capacitor C₁ increases to voltage V_(ref). at time t₁, the voltage at the negative input of comparator 11 becomes greater than (or positive relative to) that at the comparator's negative input, whereupon the output switches from its high level to its low level, as shown in voltage waveform V_(o) at time t₁. Subsequent to that time capacitor C₁ will continue to charge toward the power supply voltage V_(s).

When capacitor C₂ charges to the reference voltage level V_(ref)., at the much later time t₂, the output of comparator 12 will be switched from its low level to its high level to produce the second-occurring triggering pulse in waveform V_(T). This pulse turns on transistors T₁ and T₂ and causes capacitors C₁ and C₂ to once again quickly discharge to zero volts to start another cycle of operation. At this time t₂, comparator 11 is also switched and output V_(o) experiences a positive-going amplitude excursion. As soon as capacitor C₂ discharges at time t₂, comparator 12 immediately switches back to its low output level to terminate the triggering pulse. It is thus apparent that the measuring system oscillates to periodically repeat the waveshape of voltage V_(o) during the time interval from t₀ to t₂. In other words, the measuring system produces, at the output of comparator 11, a periodically-recurring rectangular shaped signal whose period is equal to the time duration from t₀ to t₂. The oscillating frequency, of course, is not critical. In one applicaton of the invention the frequency was around 20 kilohertz.

The time duration from t₀ to t₁, required for capacitor C₁ to charge to the reference voltage level V_(ref)., is a function of the capacitance of that capacitor. Similarly, the time duration from t₀ to t₂ is a function of the capacitance of capacitor C₂. Since the two capacitors simultaneously charge from the same low voltage level (zero volts) to the same reference voltage level and then simultaneously discharge back to the low voltage level, it can easily be proved that the ratio of the time duration from t₀ to t₁ compared to the time duration from t₀ to t₂ is equal to:

    R.sub.1 C.sub.1 /R.sub.2 C.sub.2

Hence, the duty cycle of the rectangular wave V_(o) is proportional to and represents the ratio of the capacitances of capacitors C₁ and C₂. By effectively comparing the two time durations from t₀ to t₁ and t₀ to t₂, namely by measuring the duty cycle, an output signal may be developed which represents the capacitance ratio. This is accomplished in the illustrated embodiment by means of an averaging circuit in the form of a low-pass filter comprising resistors 18 and 19 and capacitors 21 and 22. The averaging circuit 18-22 produces an output signal having an amplitude equal to the average voltage of voltage waveform V_(o), which of course is directly proportional to the duty cycle of waveform V_(o) and thereby represents the capacitance ratio of capacitors C₁ and C₂ with a high degree of accuracy. Any time that ratio changes, the amplitude of the output signal varies.

The output signal may be utilized in a variety of different ways. For simplicity it is shown applied to an indicator 23 which responds to the output signal to provide an indication or display of the value of the capacitance ratio. In most applications of the invention, however, the output signal would probably be used to control the operation of some system in response to the capacitance ratio. For example, when capacitors C₁ and C₂ are incorporated in a pressure sensor, as suggested previously, the capacitance ratio would vary as the sensed pressure changes. Since the amplitude of the output signal would now represent the sensed pressure, the signal could be employed to control a system in response to pressure, such as controlling the operation of an internal combustion engine in response to sensed intake manifold pressure.

If the capacitance of one of the capacitors C₁ and C₂ is known and it is desired to determine the size of the other capacitor, the output signal will be directly proportional to the capacitance of capacitor C₁ when capacitor C₂ is known, whereas the output signal will be inversely proportional to the capacitance of capacitor C₂ when capacitor C₁ is known.

It is to be appreciated that in the measuring system of the invention the duty cycle is immune to variations in operating characteristics, such as supply voltage variations, temperature changes, etc. If the supply voltage V_(s) changes, the reference voltage V_(ref). will vary in step and will be linearly proportional to the supply voltage whatever it is. The averaging circuit 18-22 will cancel out any undesired variations in the circuit elements, such as changes in resistors 14 and 15 which would occur in the event that they have different temperature coefficients.

Also to be fully understood is the fact that the measuring system is ratiometric in that for fixed capacitance values, the amplitude of the output signal will always be a fixed ratio of the supply voltage.

Moreover, it should be realized that the invention, in a broad sense, simultaneously stores energy in a pair of reactive devices at different storage rates such that one of the reactive devices will store energy in a given amount proportional to the reactance of the device before the other reactive device stores a given amount of energy proportional to its reactance. By comparing the two time durations required to store the two different energy amounts, it is possible to determine the ratio of the reactances of the two reactive devices. Hence, the invention could be employed to measure the inductance ratio of a pair of inductors.

While particular embodiments of the invention have been described, modifications may be made, and it is intended in the appended claims to cover all such modifications as may fall within the true spirit and scope of the invention. 

I claim:
 1. A measuring system for comparing first and second unknown capacitors to determine the ratio of their capacitances, comprising:means for simultaneously, and slowly, charging the first and second unknown capacitors, starting at time t₀, from the same relatively low voltage level to the same relatively high reference voltage level V_(ref)., each of the first and second capacitors being charged through a series-connected resistor toward the output voltage of a d-c power supply which output voltage is substantially greater than the reference voltage V_(ref)., the reference voltage being produced by a voltage divider connected across the d-c power supply and thereby being directly proportional to, and varying in step with variations of, the power supply voltage, the first capacitor having a charge time constant, determined by its capacitance and the resistance of its series-connected resistor, substantially shorter than the charge time constant of the second capacitor, which is determined by its capacitance and its series-connected resistance, so that the first capacitor charges to the reference voltage V_(ref)., at time t₁, long before the second capacitor reaches the reference voltage at the later time t₂, the time duration from t₀ to t₁ thereby being a function of the capacitance of the first capacitor while the time duration from t₀ to t₂ is a function of the capacitance of the second capacitor; means for simultaneous, and rapidly, discharging the first and second capacitors, at time t₂, back to the low voltage level by turning on a pair of substantially identical first and second transistors each of which is shunt-connected across a respective one of the first and second capacitors, the second capacitor, its shunt-connected second transistor and its series-connected resistor being included in an oscillating circuit which produces periodically-recurring triggering pulses to turn said first and second transistors on to effect rapid discharging of the first and second capacitors, the triggering pulses being produced in the oscillating circuit by a voltage comparator which compares the voltage across the second capacitor to the reference voltage V_(ref)., the period of the triggering pulses thereby being a function of the capacitance of the second capacitor and being equal to the time duration from t₀ to t₂ ; another voltage comparator which compares the voltage across the first capacitor to the reference voltage V_(ref). to detect when the first capacitor charges to the reference voltage, thereby to determine the time duration from t₀ to t₁, said other voltage comparator producing a periodically-recurring rectangular wave whose duty cycle represents the ratio of the capacitances of the first and second unknown capacitors, the rectangular wave having pulse components each of which has a pulse width equal to the time duration from t₀ to t₁ while the period of the rectangular wave is equal to the time duration from t₀ to t₂ ; an averaging circuit for producing, from said periodically-recurring rectangular wave, an output signal having an amplitude which is proportional to the ratio of the capacitances of the first and second unknown capacitors; and indicating means, responsive to said output signal, for providing an indication of the value of the capacitance ratio of the first and second unknown capacitors. 